(1) Field of the Invention
The present invention relates to a gas-discharge display panel having plural discharge cells arranged in matrix, and especially to an improvement in driving power efficiency therefor.
(2) Description of the Prior Art
Generally speaking, the various kinds of gas-discharge display panels can be classified into two types according to the form of gas-discharge, this is, a direct discharge type wherein discharge electrodes are exposed directly to a discharge space and an indirect discharge type wherein discharge electrodes are screened from the discharge space by coating them with insulation materials, so that discharge electrodes do not affect directly the gas-discharge. The gas-discharge display panel of the present invention belongs to the former type, especially, to the direct discharge type wherein plural discharge cells formed by crossing anodes and cathodes are arranged in a matrix.
The various kinds of conventional methods for displaying a color picture using the above-mentioned type of gas-discharge display panel have many insoluble problems, the most difficult of which is low driving power efficiency.
Ordinarily a discharge cell cathode of a direct discharge type gas-discharge display panel is formed from iron, nickel, or alloys of these metals and others as a wire or plate, and it generally acts as a cold cathode. Accordingly, in a conventional gas-discharge display panel of this type, the igniting voltage and the discharge maintaining voltage are fairly high, and particularly the latter exceeds 200 volts when composite gasses are used to achieve a bright color display by means of exciting a fluorescent layer with ultraviolet rays generated by the gas-discharge.
With respect to the discharging voltages of the discharge cell of the above-mentioned type, the present inventors have proposed to lower those discharging voltages by virtue of the so-called hollow cathode effect caused by the presence of a cylindrical cathode in the discharge cell, and have shown that, as a result thereof, a reduction of discharging voltages and an increase of available discharge current can be obtained. Moreover, the present inventors have also proposed the use of a cathode formed of a planar or cylindrical metal plate which is coated by materials of rare earth elements or alkali earth metals, which materials have a high electron emissivity when bombarded by ions, that is, the so-called .gamma. effect, and have shown that a reduction of discharging voltages can be obtained by using this cathode. However, even where these techniques are used, it is difficult to lower the discharge maintaining voltage below 100 volts, and furthermore, the igniting voltage is ordinarily higher than the discharge maintaining voltage by 100 volts or more, so that those voltages remain in a range from 100 volts to 200 volts. A discharge cell requiring such high driving voltages has a low efficiency of energy conversion for luminescence while requiring a high voltage driving power supply.
Generally speaking, the potential distribution along a discharge axis stretched between a cathode and anode in the discharge cell of a gas-discharge display panel is not uniform. Particularly, when a cold cathode is used, most of the driving voltage applied between the cathode and the anode is consumed between the cold cathode and a negative glow appearing in front thereof; this is called the cathode fall voltage. In most of the various kinds of gas-discharge display panels, most of the driving voltage applied between the cathode and the anode, that is, the discharging voltage, is occupied by the cathode fall voltage, which customarily reaches a level from one hundred and several score volts to three hundred volts, so that the most of discharge energy is injected concentrically in a space extending between the cathode and the negative glow. The injected energy is utilized for the electron emission caused by the bombardment of ions against the cathode, and further it is consumed for increasing, accelerating and injecting the emitted electrons into a space occupied by a positive column, as well as consumed mainly as a loss caused by the diffusion of electrons and ions into the cell wall neighbouring the cathode, and as a loss caused by heating gas molecules in a front space of the cathode. These losses of energy are, in general, a fair amount less than the amount of energy which is consumed in the front space of the cathode. Therefore, with respect to the efficiency of energy conversion from the electric power supplied to the discharge cell into ultraviolet rays, visible rays and infrared rays, particularly, into ultraviolet rays, the efficiency of the energy conversion effected in the front space of the cathode is much less than that in the space occupied by the positive column. However, in the front space of the cathode in the discharge cell of the gas-discharge display panel the noted cathode fall inevitably lowers the efficiency of energy conversion which produces a color display as the ultraviolet rays strike the fluorescent layer.
As for the cathode which exists in the discharge cell of the conventional gas-discharge display panel, a metal electrode consisting of nickel, iron or alloys of nickel and cobalt, formed into a plate, a wire or a hollow cylinder, is used, as a so-called cold cathode, from which secondary electrons are emitted by the bombardment of the positive ions in the state of a cold cathode discharge. Accordingly, the discharging voltages reach to 400 volts at the most, so that an extremely low efficiency occurs in the driving power. This low efficiency of driving power induces a rise in the temperature of the gas-discharge display panel, which results in a lowered performance and a shortened panel life caused by impure gasses being released from the discharge electrodes and other materials existing in the discharge cell. Furthermore, the usage of the gas-discharge display panel is restricted for fear that the display panel may be damaged. Moreover, the high discharging voltages restrict ones selection of available circuit elements for forming the driving circuit of the gas-discharge display panel, and impede the integration of the driving circuit, as integrated circuits typically operate with low voltages.
In order to solve the above mentioned problems, the present inventors have attempted to lower the discharging voltages of the discharge cell by means of applying electron emissive materials having a high .gamma. effect, that is, a high emissivity of secondary electrons generated by the bombardment of positive ions, to the cathode of a discharge cell. However, no significant solution has yet been obtained.
It is practicable as the second best solution to improving efficiency that a hot cathode discharge be provided in a state of low voltage arc discharge. However, even when a hot cathode is provided by externally supplying a heating current, a large area picture display panel is not practicable for the reasons discussed below.
Glass is customarily used for constructing gas-discharge display panels because of its easiness of processing, its chemical stability and its low manufacturing cost. In the gas-discharge display panel of type just described, the hot cathode of the discharge cell is supplied externally with heating current for emitting electrons. The heating of the cathode may damage the display panel by softening the glass and the life expectancy of the display panel is shortened as impure gasses contained in the glass are released. Accordingly, it is impossible to use a hot cathode heated by the externally supplied heating current in a large area gas-discharge panel.
In order to solve the problems of low energy conversion efficiency, the cathode drop voltage must be reduced, requiring the use of a hot cathode in place of the cold cathode customarily used in conventional gas-discharge display panels. However, it is not practicable to use a hot cathode supplied externally with heating current as in conventional hot cathode tubes for large area display panels. The reasons therefore are as follows.
(1) If a hot cathode supplied externally with the heating power is used in the display panel, the operating temperature increases, and the efficiency of driving power is decreased by the addition of heating power supplied to the cathodes.
(2) The heat resistivity of parts and vessels used for constructing the gas-discharge display panel is not suitable for an externally heated cathode.
(3) The number of cathodes connected in series is increased in proportion to the area of the display panel resulting in an increased voltage drop due to the increased circuit resistance. This causes an irregularity in the ignition in the discharge cells.
(4) When irregularities in ignition occur, the gas-discharge occurs locally at the series connected cathodes, and consequently an erroneous discharge is caused in the display panel.
(5) The above mentioned increase of the number of cathodes connected in series causes a rise in the heating voltage supplied to the cathodes, so that the related circuit becomes more complicated and larger than that in conventional display panels.
(6) When carbonates are applied to the hot cathode as the electron emission material, it is difficult to activate those carbonates to the exclusion of other parts and vessels provided for constructing the display panel.